LanMountainDesktop/LanMontainDesktop/App.axaml

<Application xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:sty="using:FluentAvalonia.Styling"
             x:Class="LanMontainDesktop.App"
             xmlns:local="using:LanMontainDesktop"
             RequestedThemeVariant="Default">
             <!-- "Default" ThemeVariant follows system theme variant. "Dark" or "Light" are other available options. -->

    <Application.DataTemplates>
        <local:ViewLocator/>
    </Application.DataTemplates>
  
    <Application.Styles>
        <sty:FluentAvaloniaTheme />
    </Application.Styles>
</Application>